1. Field of the Invention
This invention relates to starter motors, particularly for internal combustion engines, of the kind where the rotor of the electric motor of the starter motor drives the pinion gear wheel of the starter motor through the intermediary of an epicyclic reduction gear mechanism.
2. Description of the Prior Art
Electric starter motors for internal combustion engines incorporating an epicyclic reduction gear mechanism between the electric motor and the pinion gear wheel are known. It is also known to provide the pinion gear wheel with an overrun arrangement so that the pinion gear wheel can overrun the output shaft of the epicyclic gear mechanism, for example, when the engine being started fires and drives the pinion at a speed in excess of the epicyclic reduction gear output shaft. However, a serious problem is exhibited in the known starter motor in the event that the engine being started backfires or backrocks. In such a situation the pinion gear wheel is driven by the engine in the direction opposite to that in which it is driven by its own electric motor. Thus the overrun arrangement does not slip and the reverse rotation is transmitted back to the reduction gear mechanism of the starter motor. The gear ratio of the reduction gear mechanism may be 4.5 to 1 or even more, and thus there is considerable resistance to reverse rotation of the pinion gear wheel. Since the internal combustion engine when backfiring or backrocking generates a considerable amount of force, there is a severe risk of damage to the starter motor, for example, by twisting or breakage in torsion of the shaft carrying the pinion gear wheel and/or chipping and shearing of the teeth of the reduction gear mechanism.